1. Field of the Invention
This inveniton relates generally to a method of manufacturing an amorphous magnetic alloy, and especially to heat treatment of an amorphous magnetic alloy having high permeability.
2. Description of the Prior Art
In the art, there are known a centrifugal quenching method, single roll quenching method, double rolls quenching method and so on to prepare amorphous magnetic alloys which are known as soft magnetic material. In these methods, a melt of raw material containing metal elements and so-called forming elements is quenched to form amorphous alloy ribbons. In the method, internal stress .theta. is induced in the amorphous ribbon during manufacturing, which results in deteriorated magnetic characteristics by coupling with a magnetostriction constant .lambda.. Since the permeability .mu. satisfies the relation .mu..varies.(1/.lambda..sigma.), larger internal stress results in a deteriorated permeability .mu. and an increased coercive force Hc, and both are not desirable characteristics for soft magnetic material used as core elements of a magnetic circuit. Among various amorphous magnetic alloys, it is known that iron system amorphous alloys can be improved in permeability by annealing at the elevated temperature under an application of a magnetic field or without the application of a magnetic field to release the internal stress. However, it was found that iron-cobalt system amorphous alloys, and iron-nickel system alloys could not be improved in permeability by annealing at an elevated temperature under the application of the magnetic field or without the application of the field. Further, during processing such amorphous alloy ribbons, for example during cutting or chemical etching the ribbon to form a shaped core, stress is further induced, which results in further deteriorated magnetic characteristics, especially in permeability. In making a magnetic transducer head using these amorphous magnetic alloys as core material, a high premeability is required over an extended operating frequency range, for example 1 to 10 MH in case of a magnetic head handling a video signal. In the prior art method, the annealing is not satisfactory as mentioned above and a countermove to avoid the deterioration in permeability after the annealing is not presently available.
Considering the above, the invention of the present application proposes a method to improve magnetic characteristics of a Co-Fe system amorphous magnetic alloy, in which the amorphous alloy has a magnetic Curie temperature (Tc) lower than its crystallization temperature (Tcry). In the method an amorphous alloy ribbon can be prepared in which the ribbon has a composition of, for example, (Fe.sub.1-x Co.sub.x).sub.100-z (Si.sub.1-y By).sub.z where 0.90.ltoreq.x.ltoreq.0.98, 0.30.ltoreq.y.ltoreq.0.80 and 22.ltoreq.z.ltoreq.30. Then the ribbon is cut into a suitable core shape. The shaped core is kept at an elevated temperature T, satisfying the relation. 0.95.times.Tc(.degree.K.).ltoreq.T.sub.1 (.degree.K).ltoreq.Tcry(.degree.K) and then quenched. The method can improve magnetic characteristics, for example, permeability of the alloy over a wide frequency range. However, by this method, the frequency response characteristics is not flat which restricts the usage of the alloy, and further aging characteristics of permeability are not stable, which means that permeability becomes deteriorated during use.